/**
 * Created by Vertex on 17/3/7.
 */
/**
 * @author zz85 / https://github.com/zz85
 *
 * Based on "A Practical Analytic Model for Daylight"
 * aka The Preetham Model, the de facto standard analytic skydome model
 * http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf
 *
 * First implemented by Simon Wallner
 * http://www.simonwallner.at/projects/atmospheric-scattering
 *
 * Improved by Martin Upitis
 * http://blenderartists.org/forum/showthread.php?245954-preethams-sky-impementation-HDR
 *
 * Three.js integration by zz85 http://twitter.com/blurspline
 */

THREE.ShaderLib[ 'sky' ] = {

    uniforms: {

        luminance: { value: 1 },
        turbidity: { value: 2 },
        rayleigh: { value: 1 },
        mieCoefficient: { value: 0.005 },
        mieDirectionalG: { value: 0.8 },
        sunPosition: { value: new THREE.Vector3() }

    },

    vertexShader: [

        "uniform vec3 sunPosition;",
        "uniform float rayleigh;",
        "uniform float turbidity;",
        "uniform float mieCoefficient;",

        "varying vec3 vWorldPosition;",
        "varying vec3 vSunDirection;",
        "varying float vSunfade;",
        "varying vec3 vBetaR;",
        "varying vec3 vBetaM;",
        "varying float vSunE;",

        "const vec3 up = vec3( 0.0, 1.0, 0.0 );",

        // constants for atmospheric scattering
        "const float e = 2.71828182845904523536028747135266249775724709369995957;",
        "const float pi = 3.141592653589793238462643383279502884197169;",

        // wavelength of used primaries, according to preetham
        "const vec3 lambda = vec3( 680E-9, 550E-9, 450E-9 );",
        // this pre-calcuation replaces older TotalRayleigh(vec3 lambda) function:
        // (8.0 * pow(pi, 3.0) * pow(pow(n, 2.0) - 1.0, 2.0) * (6.0 + 3.0 * pn)) / (3.0 * N * pow(lambda, vec3(4.0)) * (6.0 - 7.0 * pn))
        "const vec3 totalRayleigh = vec3( 5.804542996261093E-6, 1.3562911419845635E-5, 3.0265902468824876E-5 );",

        // mie stuff
        // K coefficient for the primaries
        "const float v = 4.0;",
        "const vec3 K = vec3( 0.686, 0.678, 0.666 );",
        // MieConst = pi * pow( ( 2.0 * pi ) / lambda, vec3( v - 2.0 ) ) * K
        "const vec3 MieConst = vec3( 1.8399918514433978E14, 2.7798023919660528E14, 4.0790479543861094E14 );",

        // earth shadow hack
        // cutoffAngle = pi / 1.95;
        "const float cutoffAngle = 1.6110731556870734;",
        "const float steepness = 1.5;",
        "const float EE = 1000.0;",

        "float sunIntensity( float zenithAngleCos )",
        "{",
        "zenithAngleCos = clamp( zenithAngleCos, -1.0, 1.0 );",
        "return EE * max( 0.0, 1.0 - pow( e, -( ( cutoffAngle - acos( zenithAngleCos ) ) / steepness ) ) );",
        "}",

        "vec3 totalMie( float T )",
        "{",
        "float c = ( 0.2 * T ) * 10E-18;",
        "return 0.434 * c * MieConst;",
        "}",

        "void main() {",

        "vec4 worldPosition = modelMatrix * vec4( position, 1.0 );",
        "vWorldPosition = worldPosition.xyz;",

        "gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

        "vSunDirection = normalize( sunPosition );",

        "vSunE = sunIntensity( dot( vSunDirection, up ) );",

        "vSunfade = 1.0 - clamp( 1.0 - exp( ( sunPosition.y / 450000.0 ) ), 0.0, 1.0 );",

        "float rayleighCoefficient = rayleigh - ( 1.0 * ( 1.0 - vSunfade ) );",

        // extinction (absorbtion + out scattering)
        // rayleigh coefficients
        "vBetaR = totalRayleigh * rayleighCoefficient;",

        // mie coefficients
        "vBetaM = totalMie( turbidity ) * mieCoefficient;",

        "}"

    ].join( "\n" ),

    fragmentShader: [

        "varying vec3 vWorldPosition;",
        "varying vec3 vSunDirection;",
        "varying float vSunfade;",
        "varying vec3 vBetaR;",
        "varying vec3 vBetaM;",
        "varying float vSunE;",

        "uniform float luminance;",
        "uniform float mieDirectionalG;",

        "const vec3 cameraPos = vec3( 0.0, 0.0, 0.0 );",

        // constants for atmospheric scattering
        "const float pi = 3.141592653589793238462643383279502884197169;",

        "const float n = 1.0003;", // refractive index of air
        "const float N = 2.545E25;", // number of molecules per unit volume for air at
        // 288.15K and 1013mb (sea level -45 celsius)

        // optical length at zenith for molecules
        "const float rayleighZenithLength = 8.4E3;",
        "const float mieZenithLength = 1.25E3;",
        "const vec3 up = vec3( 0.0, 1.0, 0.0 );",
        // 66 arc seconds -> degrees, and the cosine of that
        "const float sunAngularDiameterCos = 0.999956676946448443553574619906976478926848692873900859324;",

        // 3.0 / ( 16.0 * pi )
        "const float THREE_OVER_SIXTEENPI = 0.05968310365946075;",
        // 1.0 / ( 4.0 * pi )
        "const float ONE_OVER_FOURPI = 0.07957747154594767;",

        "float rayleighPhase( float cosTheta )",
        "{",
        "return THREE_OVER_SIXTEENPI * ( 1.0 + pow( cosTheta, 2.0 ) );",
        "}",

        "float hgPhase( float cosTheta, float g )",
        "{",
        "float g2 = pow( g, 2.0 );",
        "float inverse = 1.0 / pow( 1.0 - 2.0 * g * cosTheta + g2, 1.5 );",
        "return ONE_OVER_FOURPI * ( ( 1.0 - g2 ) * inverse );",
        "}",

        // Filmic ToneMapping http://filmicgames.com/archives/75
        "const float A = 0.15;",
        "const float B = 0.50;",
        "const float C = 0.10;",
        "const float D = 0.20;",
        "const float E = 0.02;",
        "const float F = 0.30;",

        "const float whiteScale = 1.0748724675633854;", // 1.0 / Uncharted2Tonemap(1000.0)

        "vec3 Uncharted2Tonemap( vec3 x )",
        "{",
        "return ( ( x * ( A * x + C * B ) + D * E ) / ( x * ( A * x + B ) + D * F ) ) - E / F;",
        "}",


        "void main() ",
        "{",
        // optical length
        // cutoff angle at 90 to avoid singularity in next formula.
        "float zenithAngle = acos( max( 0.0, dot( up, normalize( vWorldPosition - cameraPos ) ) ) );",
        "float inverse = 1.0 / ( cos( zenithAngle ) + 0.15 * pow( 93.885 - ( ( zenithAngle * 180.0 ) / pi ), -1.253 ) );",
        "float sR = rayleighZenithLength * inverse;",
        "float sM = mieZenithLength * inverse;",

        // combined extinction factor
        "vec3 Fex = exp( -( vBetaR * sR + vBetaM * sM ) );",

        // in scattering
        "float cosTheta = dot( normalize( vWorldPosition - cameraPos ), vSunDirection );",

        "float rPhase = rayleighPhase( cosTheta * 0.5 + 0.5 );",
        "vec3 betaRTheta = vBetaR * rPhase;",

        "float mPhase = hgPhase( cosTheta, mieDirectionalG );",
        "vec3 betaMTheta = vBetaM * mPhase;",

        "vec3 Lin = pow( vSunE * ( ( betaRTheta + betaMTheta ) / ( vBetaR + vBetaM ) ) * ( 1.0 - Fex ), vec3( 1.5 ) );",
        "Lin *= mix( vec3( 1.0 ), pow( vSunE * ( ( betaRTheta + betaMTheta ) / ( vBetaR + vBetaM ) ) * Fex, vec3( 1.0 / 2.0 ) ), clamp( pow( 1.0 - dot( up, vSunDirection ), 5.0 ), 0.0, 1.0 ) );",

        //nightsky
        "vec3 direction = normalize( vWorldPosition - cameraPos );",
        "float theta = acos( direction.y ); // elevation --> y-axis, [-pi/2, pi/2]",
        "float phi = atan( direction.z, direction.x ); // azimuth --> x-axis [-pi/2, pi/2]",
        "vec2 uv = vec2( phi, theta ) / vec2( 2.0 * pi, pi ) + vec2( 0.5, 0.0 );",
        "vec3 L0 = vec3( 0.1 ) * Fex;",

        // composition + solar disc
        "float sundisk = smoothstep( sunAngularDiameterCos, sunAngularDiameterCos + 0.00002, cosTheta );",
        "L0 += ( vSunE * 19000.0 * Fex ) * sundisk;",

        "vec3 texColor = ( Lin + L0 ) * 0.04 + vec3( 0.0, 0.0003, 0.00075 );",

        "vec3 curr = Uncharted2Tonemap( ( log2( 2.0 / pow( luminance, 4.0 ) ) ) * texColor );",
        "vec3 color = curr * whiteScale;",

        "vec3 retColor = pow( color, vec3( 1.0 / ( 1.2 + ( 1.2 * vSunfade ) ) ) );",

        "gl_FragColor.rgb = retColor;",

        "gl_FragColor.a = 1.0;",
        "}"

    ].join( "\n" )

};

THREE.Sky = function () {

    var skyShader = THREE.ShaderLib[ "sky" ];
    var skyUniforms = THREE.UniformsUtils.clone( skyShader.uniforms );

    var skyMat = new THREE.ShaderMaterial( {
        fragmentShader: skyShader.fragmentShader,
        vertexShader: skyShader.vertexShader,
        uniforms: skyUniforms,
        side: THREE.BackSide
    } );

    var skyGeo = new THREE.SphereBufferGeometry( 450000, 32, 15 );
    var skyMesh = new THREE.Mesh( skyGeo, skyMat );

    // Expose variables
    this.mesh = skyMesh;
    this.uniforms = skyUniforms;

};